Electroplating is a common process for depositing a thin film of metal or alloy on a workpiece article such as various electronic components for example. In electroplating, the article is placed in a suitable electrolyte bath containing ions of a metal to be deposited. The article forms a cathode, which is connected to the negative terminal of a power supply, and a suitable anode is connected to the positive terminal of the power supply. Electrical current flows between the anode and cathode through the electrolyte, and metal is deposited on the article by an electrochemical reaction.
Electroplating is widely used in the thin film head industry to fabricate magnetic and non-magnetic materials that constitute the writing part of a read-write head. Magnetic materials with Nickel and Iron are widely used as the write pole (and read shield) materials in thin film heads. Different compositions of nickel and iron provide different properties and hence are suitable for different applications. Other plating materials include cobalt-iron compositions.
During plating, it is desirable to obtain the purest volume of magnetic material possible. If impurities such as iron hydroxide or iron oxide are present during plating, the purity of the resulting magnetic material is reduced, with a resulting reduction in the maximum flux obtainable.
The current state of the art has shifted towards material with a high iron content and the resulting high magnetic moment. To raise the iron content in the deposit, however, more iron must be used in the plating solution. More iron in the bath means more ferric ions (Fe3+). The ferric ion content of plating baths containing iron can adversely influence both the rate and nature of the metal deposits.
Ferric ions are created by oxidation of ferrous iron (Fe2+) in the plating solution. For example, air oxidation of the ferrous iron results in a continuing buildup of ferric ion in the plating solution. Ferrous ions can also react with dissolved oxygen in the plating solution to form ferric ions.
Ferric ions are harmful in that they can form iron hydroxide or iron oxide, which precipitates as particulate matter. Particulate matter, as known to those skilled in the art, affects the purity of the plating deposits, and thus its magnetic characteristics.
Ferric ions also affect the rate of plating. Ferric ions react with electrons at the wafer surface and return (are reduced) to ferrous ions. This consumes power, reducing current efficiency. The result is inconsistent quality and quantity, as the amount of electrons consumed for this side reaction will vary with the concentration of Fe3+ in the bath. For example, assume the plating bath is used regularly on a daily basis, but is left idle for a period of time. A high level of Fe3+ will have formed over the idle period due to the prolonged exposure of the plating solution to air and lack of electrolytic reduction of Fe3+. Thus, the level of Fe3+ when plating is resumed will be much higher than the level at which plating was discontinued. Consequently, the current efficiency changes with idle time due to the variation in current being used up for ferric reduction. When Fe3+ reduces on the wafer surface, products of the reaction may become incorporated in the wafer structure. In addition, when plating Ni and Fe, ferric hydroxide particles are suspended in the plating solution. Those can also get incorporated, which rapidly reduces magnetic film quality.
The prior art has made many attempts to control the ferric content in plating solutions. The usual practice is to allow the ferric to build up until it precipitates. The precipitate is continuously collected on a sub-micron filter through which the plating solution is circulated. One disadvantage of this approach is that the filter quickly becomes clogged. Further, the ferric ion content is always high, i.e., at saturation, and thus the problems mentioned above remain present.
Another practice is to introduce a completion agent to keep the ferric ions in soluble form, and avoid precipitation. One drawback to this method is that the ferric content continues to build up over time, resulting in an increase in ferric ion reaction on the wafer. The current efficiency and therefore the plating rate thus decrease over time.
Another practice used to mitigate the ferric problem is to blanket the bath with nitrogen to prevent the air oxidation of the ferrous ions. This is not completely successful, because the bath is circulated out to plating cells which cannot conveniently be operated under a nitrogen blanket.
The potentiostatic reduction of the ferric has also been employed, but it requires complex instrumentation including a reference electrode, and a sacrificial anode which will not cause the oxidation of ferrous ions to ferric ions.
What is therefore needed is a way to not only reduce the ferric ion content in a plating bath, but also a way to do so efficiently.